Studio Technologies 5120 2011 - Manual (Page 13)

Model 5120 User Guide Issue 3, November 2011
Studio Technologies, Inc. Page 13
Model 5120
Line/IFB Output Module
Cabling Issues – Crosstalk
The Model 5120’s IFB output conforms to
a broadcast-industry standard for sending
DC power and two channels of audio over
a single pair with shielded audio cable. This
implementation allows standard portable
cables, such as are used for microphone
signals, to interconnect various IFB user
devices. This method is undoubtedly con-
venient and practical, but is not without
limitations. The main audio quality issue is
the possibility of crosstalk between the two
audio channels. This issue arises due to the
capacitance presented by the two wires that
form the twisted pair. The greater the capac-
itance presented and the longer the cable
run, the greater the crosstalk will become. Is
this normally a problem during actual use?
No. But it’s something that should be noted.
Superior Power Delivery and
Audio Quality
One of the Model 5120’s strengths is its
ability to very effectively deliver energy to
the connected IFB user devices. This allows
the devices to be supported over longer
cable runs. How does the Model 5120 ac-
complish this? Simply by having circuitry
that is superior to that used in most of the
“industry-standard” equipment. In most
IFB interface devices, an adjustable volt-
age regulator integrated circuit is used as a
combination of audio modulator and current
limiter. While this is a simple and inexpen-
sive solution, it’s not without significant limi-
tations. The major problem with this method
is the type of voltage-current “knee” that is
created. As the load current increases past
about 50 percent of the rated maximum
the output voltage begins to decrease. This
means that the usable power delivered to
the connected device(s) will start to drop
well before the rated output is reached. This
limitation will become significant in applica-
tions that use long cable runs. As the IFB
circuit voltage begins to drop problems with
user device performance can occur.
Contrast this situation with the performance
provided by the Model 5120. The DC volt-
age supplied by its IFB circuit won’t “poop
out” when loaded over its entire 0 to 120
milliampere range. This will allow IFB belt-
pack devices to work correctly in many
more applications. Figure 5 shows the IFB
circuit voltage-current curves for the RTS
4000-series and the Model 5120’s IFB out-
put. The performance differences are quite
interesting. While the RTS Model 4010 is
rated to support a much higher maximum
current (versus the Model 5120) it doesn’t
effectively deliver it. The Model 5120’s
output voltage varies little over its rated
output current range. Not so with the RTS
4010. Well before its maximum rated
current is reached its output voltage falls
off precipitously.
It’s also interesting to note the reason why
typical IFB circuit audio quality is less than
pristine. It’s not hard to notice the back-
ground “hiss” that is seemingly always
present on pin 2 (DC with channel 1 audio)
of the interface connector. Technically, it’s
white noise that comes from the adjustable
voltage regulator being used as an “AM”
modulator and current limiter. The noise is
an artifact of the design topology and sim-
ply can’t be overcome. How does Studio
Technologies know this? Because our first
“breadboard” designs used this method and
achieved the same poor results! Only after
the problem came to light did work on an
improved circuit begin. The result was well
worth the effort.